Organic electroluminescent (EL) devices are ideal candidates for use in portable display applications because of their low power drain and capability of a full range of colors.
A typical device consists of thin layers of organic molecules sandwiched between transparent and metallic electrodes. Under an applied bias, oppositely charged carriers are injected from the opposing contacts and are driven through the device by the electric field. Some of these oppositely charged carriers capture one another within the emissive layer to give out light at a wavelength corresponding to the energy gap of the organic emissive materials. In order to achieve high EL efficiency, it is necessary to balance the rates of injection of electrons and holes from opposite contacts into the device. In most case, the electron injection has proved to be more difficult then hole injection because of the relative large energy barrier existing at the n-contact and organic interface. To lower the energy barrier for efficient electron injection, often metals with low work function such as calcium, magnesium, etc. are needed as the electron-injecting contact. An alternative way to lower the energy barrier for efficient electron injection is to use an organic material of high electron affinity at the metal-organic interface. An organic material of high electron affinity has low "Lowest-Unoccupied-Molecular-Orbit" (LUMO) energy level that reduces the energy barrier for electron injection at the metal-organic interface, thus increasing the electron injection rate, resulting in a device of high efficiency and low working voltage.
In the prior art, a class of organic materials that have exhibited high EL efficiency in devices, are those based on metal complexes of 8-hydroxyquinoline and its derivatives (Vanslyke et al U.S. Pat. Nos. 4,539,507; 5,150,006). Another class of organometallic complexes that have also resulted in highly efficient organic EL devices is disclosed in a copending U.S. Patent Application entitled "NEW ORGANOMETALLIC COMPLEXES FOR USE IN LIGHT EMITTING DEVICES", filed 12 Sep. 1994, bearing Ser. No. 08/304,451, and assigned to the same assignee.
It is a purpose of this invention to provide a class of new organometallic complexes with high electron affinities for use in light emitting devices.
It is another purpose of the present invention to provide a class of new organometallic complexes for emission in the green range in light emitting devices.
It is a further purpose of this invention to provide preparation methods for the disclosed organometallic complexes for use in light emitting devices.